Liquid epoxy resin composition containing wollastonite and calcite

ABSTRACT

Epoxy resin casting compositions comprising 
     (a) a liquid aromatic glycidyl or β-methylglycidyl ether or a liquid mixture of several aromatic glycidyl or β-methylglycidyl ethers containing on average more than one glycidyl ether group per molecule, 
     (b) one or more than one acid hardener for the glycidyl ether, 
     (c) a curing accelerator, 
     (d) a filler mixture containing calcite and wollastonite in an amount from 40 to 60% by weight, based on components (a), (b) and (d), and as optional components, 
     (e) customary additives for epoxy resin casting compositions are suitable in particular for coating or encapsulating electrical or electronic components and, more particularly, electrical ignition coils.

This is a continuation-in-part application of the InternationalApplication PCT/EP 95/02345 with an international filing date of Jun.17, 1995.

The present invention relates to an epoxy resin casting compositioncontaining calcite and wollastonite as fillers and which is suitable foruse in particular for coating or encapsulating electrical or electroniccomponents and, more particularly, electrical ignition coils.

Epoxy resin casting compositions for coating or encapsulating electricalor electronic components are known. EP-A-0 348 193, inter alia,discloses an epoxy resin casting composition loaded with an inorganicfiller for encapsulating ignition coils and, in addition, a polyetherpolyol to prevent, in particular, corrosion of the coil and of thewinding. However, polyether polyols have the drawback of lowering theglass transition temperature (Tg value) of the epoxy resin castingcomposition.

DE-OS 32 29 558 discloses epoxy resin casting compositions loaded withchalk for impregnating or casting electrical components. However, whilethe cured epoxy resin casting compositions have a higher Tg value, theyare often too brittle and do not meet the requirements made of them whensubjected to the temperature shock test. Moreover, they comprise a blendof aromatic and cycloaliphatic epoxy resin which may lead totoxicological problems.

The epoxy resin casting compositions disclosed in EP-A-0540 467containing quartz powder as filler and a polyether polyol, also do notmeet the requirements when subjected to the temperature shock test.

Surprisingly, it has now been found that the above-mentioned drawbacksof an epoxy resin casting composition based on aromatic glycidyl ethersmay be avoided by adding to this composition a filler mixture containingcalcite and wollastonite in specific amounts.

Accordingly, the object of this invention is to provide an epoxy resincasting composition comprising

(a) a liquid aromatic glycidyl or β-methylglycidyl ether or a liquidmixture of several aromatic glycidyl or β-methylglycidyl etherscontaining on average more than one glycidyl ether group per molecule,

(b) one or more than one acid hardener for the glycidyl ether,

(c) a curing accelerator,

(d) a filler mixture containing calcite and wollastonite in an amountfrom 40 to 60% by weight, based on components (a), (b) and (d), and asoptional components,

(e) customary additives for epoxy resin casting compositions.

Suitable components (a) are all aromatic glycidyl or β-methylglycidylethers as well as mixtures of several aromatic glycidyl orβ-methylglycidyl ethers, provided they remain liquid at temperatures ofup to c. 50° C. This means that it is also possible to use mixtures ofliquid and solid glycidyl or β-methylglycidyl ethers as well as mixturesof mono- and di(β-methyl)glycidyl ethers or of mono- andpoly(β-methyl)glycidyl ethers, provided that a liquid mixture isobtained for component (a) and that the glycidyl compounds in thismixture contain on average more than one glycidyl group in the molecule.

Such glycidyl ethers are known and are obtainable, for example, byreacting a compound containing at least one or more than one freephenolic hydroxyl group with epichlorohydrin or β-methylepichlorohydrinunder alkaline conditions or in the presence of an acid catalyst and bysubsequent treatment with alkali.

The glycidyl ethers of this type are typically derived from mononuclearphenols, typically from phenol, 2-methylphenol, 4-tert-butylphenol,resorcinol or hydroquinone, or they are based on polynuclear phenols,typically bis(4-hydroxyphenyl)methane, 4,4'-dihydroxybiphenyl,bis(4-hydroxyphenyl)sulfone, 1,1,2,2-tetrakis(4-hydroxyphenyl)ethane,2,2-bis(4-hydroxyphenyl)propane,2,2-bis(3,5-dibromo-4-hydroxyphenyl)propane, as well as the novolaksobtainable by condensation of aldehydes, typically formaldehyde,acetaldehyde, chloral or furfuraldehyde, with phenols such as phenol, orwith phenols substituted in the nucleus by one or more than one chlorineatom or by one or more than one C₁ -C₉ alkyl group, for example4-chlorophenol, 2-methylphenol or 4-tert-butylphenol, or by condensationwith bisphenols such as those of the indicated type.

It is preferred to use glycidyl ethers having an epoxy content from 1 to10 equivalents/kg for the novel epoxy resin casting compositions.

Component (a) of the inventive epoxy resin casting compositions ispreferably a liquid diglycidyl ether of bisphenol A or a liquiddiglycidyl ether of bisphenol F.

Suitable components (b) are usually the polycarboxylic acids and theanhydrides thereof, typically aliphatic polycarboxylic acids such asmaleic acid, oxalic acid, succinic acid, nonyl- or dodecylsuccinic acid,glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid ordimerised or trimerised linoleic acid, cycloaliphatic polycarboxylicacids such as tetrahydrophthalic acid,methylenedomethylenetetrahydrophthalic acid,hexachloroendomethylenetetrahydrophthalic acid,4-methyltetrahydrophthalic acid, hexahydrophthalic acid, or4-methylhexahydrophthalic acid, or aromatic polycarboxylic acids such asphthalic acid, isophthalic acid, terephthalic acid, trimellitic acid,pyromellitic acid or benzophenone-3,3',4,4'-tetracarboxylic acid, aswell as the anhydrides of the cited polycarboxylic acids.

Component (b) of the novel epoxy resin casting compositions ispreferably a polycarboxylic anhydride.

In particular, component (b) of the novel epoxy resin castingcompositions is a cycloaliphatic polycarboxylic anhydride.

The amount of the hardener (b) will depend on the chemical nature of thehardener and on the desired properties of the curable mixture and thehardened product. Typically from 0.4 to 1.1 equivalents of carboxylgroup or anhydride group are used per 1 epoxy equivalent of the glycidylether (a).

Suitable curing accelerators (c) are typically tertiary amines or thesalts thereof, such as 2,4,6-tris(dimethylaminoethyl)phenol and otherMannich bases, N-benzyldimethylamine and triethanolamine, imidazoles,e.g. 1-methylimidazole, quaternary ammonium salts, e.g.benzyltrimethylammonium chloride, alkali metal alkoxides or phosphoniumsalts, e.g. tetraphenylphosphonium bromide.

Component (c) of the inventive epoxy resin casting composition ispreferably a tertiary amine, a quaternary ammonium salt, an imidazole ora phosphonium salt.

In its pure form, the calcite used in the novel epoxy resin castingcomposition as filler consists of colourless transparent crystals and iscommercially available in various particle sizes. The epoxy resincasting compositions of this invention preferably contain a calcitehaving a particle size of less than 10 μm.

The wollastonite used as further filler in the inventive epoxy resincasting compositions is a naturally occuring calcium silicate of formulaCa₃ Si₃ O₉ ! of needle-like shape and having a particle size in themicron range. The artificially manufactured wollastonite is also ofneedle-like shape. Wollastonite is commercially available, e.g. from OyPartek.

The filler mixture (d) of the novel epoxy resin casting compositionspreferably has a particle size of less than 25 μm. More particularly,this filler mixture (d) has a particle size of less than 20 μm.

Customary additives (e) for epoxy resin casting compositions which maybe added to the novel epoxy resin casting compositions are furtherfillers such as quartz powder, aluminium hydroxide, aluminium oxide,calcium carbonate, further calcium silicates, mica, glass fibres, glasspowder or dolomite, pigments or dyes such as titanium oxide, carbonblack or iron oxide black, flame retardants, thixotropic agents, flowcontrol agents such as silicones, silicone oil, waxes or stearates, someof which also find utility as mould release agents, e.g.γ-glycidyloxipropyltrimethoxysilane, antioxidants or light stabilisers,the amount of additives in the inventive epoxy resin castingcompositions being altogether not more than 10% by weight, based oncomponents (a) and (b).

Component (e) of the novel epoxy resin casting compositions ispreferably a sheet silicate modified by a quaternary organic ammoniumsalt, more particularly based on the mineral clay bentonite,commercially available under the registered trademark Bentones®,supplied by NL Chemicals.

The novel epoxy resin casting compositions can be prepared inconventional manner by mixing the components with known mixingaggregates, typically stirrers, kneaders or roll mills.

The epoxy resin casting compositions of this invention are liquid atambient temperature and have very good impregnating as well asantideposition and deaerating properties. The ignition coilsencapsulated with the novel epoxy resin casting compositions typicallyshow excellent impregnation of the fine wire winding of the secondarycoil.

The novel epoxy resin casting compositions can be cured in known mannerby heating.

The heating step can also be carried out stepwise. Curing is usuallyeffected by heating to the temperature range from 80° to 200° C.,preferably from 100° to 180° C.

The components encapsulated with the fully cured novel epoxy resincasting compositions are distinguished by high resistance to thermalageing and good tear resistance, in particular in the durability testand in the temperature shock test.

By virtue of their excellent stability to thermal and mechanical stress,the fully cured novel epoxy resin casting compositions are particularlysuitable for utilities in vacuum casting resin technology and, moreparticularly, for encapsulating electrical or electronic components.

Accordingly, the invention also relates to the use of the novel epoxyresin casting compositions for encapsulating electrical or electroniccomponents and, in particular, for encapsulating electrical ignitioncoils.

EXAMPLE 1

34.15 g of liquid diglycidyl ether of bisphenol A (epoxy value from 5.2to 5.4 equivalent/kg; viscosity from 1000 to 1200 mPa.s), 0.05 g ofsilicone oil (Silicone SH 5500, supplied by Toray Industries), 0.3 g ofγ-glycidyloxipropyltrimethoxysilane (Silan® A-187, supplied by UnionCarbide Chemicals) and 0.5 g of iron oxide black are mixed. To thismixture are added, as fillers, in increments and with stirring, 24.9 gof calcite having a particle size from 5-7 μm, density 50% (Juraweiss,Gelbsiegel, supplied by Ulmer Fullstoffvertrieb), 40 g of wollastonite(Wollastonit FW 200, supplied by Oy Partek) and 0.1 g of Bentone® SD-2(bentonite which is modified by a quaternary organic ammonium salt,supplied by NL Chemicals). The loaded epoxy resin is blended with 28.83g of methylhexahydrophthalic anhydride and 0.17 g of 1-methylimidazoleto give a low viscosity reaction resin composition (RRC) as castingresin, having the following properties:

viscosity at 60° C. (DIN 16945) =500 mPa.s

gel time at 90° C. (DIN 16945) =60 min.

Before the ignition coils are encapsulated with the casting resin, theyare preheated to above 100° C. over at least 2 hours and then evacuatedat 1 mbar for 2 minutes. The RRC is degassed in a storage vessel at 60°C. and 1 mbar. The ignition coils are then encapsulated in known mannerby the vacuum casting technique at 4 mbar. Subsequently, theencapsulation of the ignition coils is cured in an oven, first at 90° C.for 1.5 hours and then at 120° C. for a further 2 hours. The mouldingsso obtained have the following properties:

glass transition temperature (DSC*) =135°-140° C.

flexural strength (ISO 178) =90 N/mm²

modulus of elasticity (ISO 178) =7600 N/mm²

flexural elongation (ISO 178) =1.4%

linear thermal coefficient

of expansion (TMA**) =40 ppm/K.

*) DSC =differential scanning calorimeter

**) TMA =thermomechanical analysis

The ignition coils encapsulated with the fully cured casting resinwithstand the following tests:

temperature shock test, after one hour from -40° C. to 120° C. and afterone hour from 120° C. to -40° C. over 400 cycles, i.e. over 800 hours.Durability test at 140° C. ambient temperature over more than 1000hours.

If the ignition coils encapsulated with the fully cured casting resinare cut through and the cut surfaces are then polished, it is found thatthe windings of the primary and secondary coils are completelyimpregnated with the casting resin.

EXAMPLE 2

34.15 g of a liquid diglycidyl ether of bisphenol F (epoxy value from5.5 to 5.8 equivalents/kg), 0.05 g of silicone oil (Silicone SH 5500,supplied by Toray Industries), 0.3 g ofγ-glycidyloxipropyltrimethoxysilane and 0.5 g of iron oxide black aremixed. To this mixture are added, as fillers, in increments and withstirring, 24.9 g of calcite having a particle size from 5 to 7 μm(Juraweiss), 40.0 g of wollastonite FW 200 and 0.1 g of Bentone® SD-2.The loaded epoxy resin is blended with 29.82 g ofmethylhexahydrophthalic anhydride and 0.18 g of 1-methylimidazole togive a low viscosity reaction resin composition as casting resin, havingthe following properties:

viscosity at 60° C. (DIN 16945)=350 mPa.s

gel time at 90° C. (DIN 16945)=55 min.

The ignition coils are encapsulated with the casting resin in generalaccordance with the technology employed in Example 1. The mouldings soobtained have the following properties:

glass transition temperature (DSC) =130°-135° C.

flexural strength (ISO 178) =100 N/mm²

modulus of elasticity (ISO 178) =7500 N/mm²

flexural elongation (ISO 178) =1.4%

linear thermal coefficient of

expansion (TMA) =40 ppm/K.

The ignition coils encapsulated with the casting resin withstand thetests described in Example 1 and show complete impregnation of thewindings.

EXAMPLE 3

34.15 g of a liquid diglycidyl ether of bisphenol A (epoxy value from5.2 to 5.4 equivalents/kg), 0.05 g of silicone oil (Silicone SH 5500,supplied by Toray Industries), 0.3 g ofγ-glycidyloxipropyltrimethoxysilane and 0.5 g of iron oxide black aremixed. To this mixture are added, as fillers, in increments and withstirring, 40.0 g of calcite having a particle size from 5 to 7 μm(Juraweiss), 24.9 g of wollastonite FW 200 and 0.1 g of bentone® SD-2.The loaded epoxy resin is blended with 29.82 g ofmethylhexahydrophthalic anhydride and 0.18 g of 1-methylimidazole togive a low viscosity reaction resin composition as casting resin, havingthe following properties:

viscosity at 60° C. (DIN 16945) =400 mPa.s

gel time at 90° C. (DIN 16945) =55 min.

The ignition coils are encapsulated with the casting resin in generalaccordance with the technology employed in Example 1. The mouldings soobtained have the following properties:

glass transition temperature (DSC) =130°-135° C.

flexural strength (ISO 178) =100 N/mm²

modulus of elasticity (ISO 178) =7500 N/mm²

flexural elongation (ISO 178) =1.4%

linear thermal coefficient of

expansion (TMA)=40 ppm/K.

The ignition coils encapsulated with the casting resin withstand thetests described in Example 1 and show complete impregnation of thewindings. In addition, the cured casting resin composition shows nosedimentation whatever and the fillers are homogeneously distributedthroughout the encapsulation, as may be found by determining the ashcontent of samples taken from different sites of the encapsulation.

EXAMPLE 4

34.45 g of a liquid diglycidyl ether of bisphenol A (epoxy value from5.2 to 5.4 equivalents/kg), 0.05 g silicone oil (Silicone SH 5500,supplied by Toray Industries) and 0.5 g of colour paste, commerciallyavailable under the registered trademark Araldite® DW 0137, supplied byCiba-Geigy, are mixed. To this mixture are added, as fillers, inincrements and with stirring, 40.0 g of calcite having a particle sizefrom 5 to 7 μm (Juraweiss), 24.8 g of wollastonite FW 200 and 0.1 g ofbentone® SD-2. The loaded epoxy resin is blended with 20.18 g ofhexahydrophthalic anhydride, 8.65 g of methylhexahydrophthalic anhydrideand 0.17 g of 1-methylimidazole to give a low viscosity reaction resincomposition as casting resin, having the following properties:

viscosity at 60° C. (DIN 16945) =450 mPa.s

gel time at 90° C. (DIN 16945) =60 min.

The ignition coils are encapsulated with the casting resin in generalaccordance with the technology employed in Example 1. The mouldings soobtained have the following properties:

glass transition temperature (DSC) =135°-140° C.

flexural strength (ISO 178) =95 N/mm²

modulus of elasticity (ISO 178) =7600 N/mm²

flexural elongation (ISO 178) =1.4%

linear thermal coefficient of expansion (TMA) =40 ppm/K.

The ignition coils encapsulated with the fully cured casting resinwithstand the following tests:

temperature shock test, after one hour from -40° C. to 120° C. and afterone hour from 120° C. to -40° C. over 600 cycles, i.e. over 1200 hours.

Durability test at 140° C. ambient temperature over more than 2000hours.

On cutting through the ignition coils encapsulated with the fully curedcasting resin and polishing the cut surfaces, it is found that thewindings of the primary and secondary coils are completely impregnatedwith the casting resin. The fully cured casting resin composition showsa homogeneous distribution of the fillers as may be found by determiningthe ash content of different samples of the moulding.

What is claimed is:
 1. An epoxy resin casting composition that is liquidat ambient conditions consisting essentially of(a) a liquid epoxy resincomponent selected from the group consisting of glycidyl ethers ofphenolic compounds containing on average more than one glycidyl ethergroup per molecule, β-methylglycidyl ethers of phenolic compoundscontaining on average more than one glycidyl ether group per molecule,and mixtures thereof, (b) one or more than one acid hardener for theresin component, (c) a curing accelerator, (d) a filler mixturecontaining calcite and wollastonite in an amount from 40 to 60% byweight, based on components (a), (b) and (d), and as optionalcomponents, wherein the weight ratio of calcite to wollastonite is inthe range of about 38:62 and 62:38, (e) customary additives for epoxyresin casting compositions selected from group consisting of quartzpowder, aluminum hydroxide, aluminum oxide, mica, glass fibers, glasspowder, dolomite, pigments, dyes, flame retardants, thixotropic agents,flow control agents, antioxidants, light stabilizers and sheet silicate,which is modified by quaternary organic ammonium salt.
 2. An epoxy resincasting composition according to claim 1, wherein component (a) is aliquid diglycidyl ether of bisphenol A or a liquid diglycidyl ether ofbisphenol F.
 3. An epoxy resin casting composition according to claim 1,wherein component (b) is a carboxylic anhydride.
 4. An epoxy resincasting composition according to claim 1, wherein component (b) is acycloaliphatic carboxylic anhydride.
 5. An epoxy resin castingcomposition according to claim 1, wherein component (c) is a tertiaryamine, a quaternary ammonium salt, an imidazole or a phosphonium salt.6. An epoxy resin casting composition according to claim 1, wherein thefiller mixture (d) has a particle size of less than 25 μm.
 7. An epoxyresin casting composition according to claim 1, wherein the fillermixture (d) has a particle size of less than 20 μm.
 8. An epoxy resincasting composition according to claim 1, comprising a component (e)which contains a sheet silicate which is modified by a quaternaryorganic ammonium salt.